It is known in the art of high pressure fluid or supercritical fluid restrictors to restrict the flow of fluid with fluid restrictors instead of valves because the fluid flow rates, which can be in the range of 0 ml/min. to 10 ml/min., are too low for accurate and reproducible flow control by conventional valves.
In one type of fluid restrictor, holes are formed in a circular disc that are supported in the path of the flowing fluid by the necessary hardware, seals and fittings. A problem exists with such fluid restrictors in that the small size of the hole and its abrupt change in cross sectional area allow it to become clogged easily. This is an undesirable event in that it typically entails dismounting the fluid restrictor so that it can be cleaned or replacing it altogether. A further disadvantage is that the fluid restriction cannot be adjusted without mechanically changing the entire restrictor.
It is also known to use linear fused silica tubing having a small internal channel as a fluid restrictor. Tapered fused silica restrictors are also formed by heating and drawing out the fused silica tubing until the internal channel is the appropriate dimension. (T. L. Chester, D. P. Innis, G. D. Owens, Analytical Chemistry, 57 (1985) 2243-2247).
"Integral" fused silica restrictors (E. J. Guthrie, H. E. Schwartz, J. Chromatographic Science, 24 (1986) 236-241) are formed by heating the end of the fused silica tubing until it melts shut. Then, the end is filed or sanded until a small hole is formed. Ceramic or alumina frits have also been formed at the ends of the fused silica restrictor (K. E. Morkides, S. M. Fields, M. L. Lee, J. Chromatographic Science, 24 (1986) 254-257 and U.S. Pat. No. 4,793,920). A review of restrictors is given by B. W. Wright, R. D. Smith, "Restrictor Performance Characteristics for SFC", Chapter 10 of Book Modern Supercritical Fluid Chromatography, Ed C. M. White, Dr. Alfred Huthig Verlag, ISBN 3-7785-1569-1, 1988.
A length of fused silica tubing does better as a restrictor than a hole in a disc as it plugs less often because the internal diameter can be larger, but it is limited by other deficiencies including 1) it is brittle and can break easily, 2) a long length must be used, typically 10 to 100 centimeters which makes its use inconvenient to the user, and 3) they are difficult to heat. Such restrictors also are lacking in that they cannot be adjusted. Similarly, U.S. Pat. No. 4,776,618 shows a high pressure coupling which is not adjustable.
European Patent No. 89121728.3 discloses another type of fluid restrictor which is based on a valve which is disposed within the fluid flow to constrict it. As an aside, although not directed to the art of supercritical fluid restriction, there is U.S. Pat. No. 2,532,452 which discloses an essentially adjustable tubular fluid flow restrictor for refrigeration systems.
Accordingly, there is a need to overcome plugging due to particles collecting in the restrictor area, and to compensate for changes in viscosity due to changes in the composition of the fluid such as the addition of water, modifiers, etc. and to provide means so that fluid flow can be easily adjusted to desired values.
The present invention provides an adjustable supercritical fluid restrictor wherein an elastic tube is adjustably compressed such that the channel within is restricted a desired amount.